The skin is mainly divided into the epidermis, dermis, and subcutaneous tissue. The epidermis is further divided into, from the surface, the sebaceous membrane, stratum corneum, stratum granulosum, stratum spinosum, and stratum basale.
The sebaceous membrane consists of the sebum secreted from the sebaceous gland, the sweat gland, or the like. Sebum plays a role in preventing water loss. Transepidermal water loss (TEWL) means the amount of water that is volatilized unconsciously from the body through the stratum corneum. The amount of sebum and TEWL are inversely correlated; for example, if the amount of sebum is large, TEWL is decreased. A higher TEWL value means decreased barrier function of the stratum corneum. As a result, the skin becomes grainy or dry, resulting in rough skin. If secretion of sebum is too much, the skin becomes oily and sticky and the sebum is changed to an irritant, which is a factor for inflammation and acne of the skin. Moderate secretion of sebum is needed so that the sebaceous membrane can perform an important function.
In the stratum corneum under the sebaceous membrane, corneocytes filled with hard keratin proteins are stacked in about 1 mm layers. Each corneocyte is linked by intercellular lipids. The structure of the stratum corneum is so compact that water and fluid in the body do not leak out. The phenomenon that basal cells divide once when corneocytes in the top layer of the stratum corneum is shed is referred to as keratinization. As a result of the fact that old keratin is shed to be replaced by a new corneocyte, the functions of the stratum corneum are maintained and the thickness of the epidermis is kept constant.
Even when corneocytes are formed by hard keratin proteins, softness of the skin needs to be maintained to some extent. To do this, the stratum corneum contains 20% to 30% of moisture. If moisture is lost, keratin becomes hard and the skin becomes dry.
Natural moisturizing factor (NFM) plays a role in retaining the moisture in the stratum corneum. NFM consists of amino acid, PCA (pyrrolidone carboxylic acid), lactate, urea, and the like. NMF having high hydrophilicity is helpful in retaining moisture.
Intercellular lipids are lipids formed by keratinization of keratinocytes. Intercellular lipids consist of ceramides, cholesterol, cholesteryl ester, and the like. Ceramide and free fatty acids such as linoleic acid contained therein play a role in retaining moisture and the barrier function of the stratum corneum. If an abnormality occurs in intercellular lipids, the water-retaining function is lost, the skin becomes dry, and inflammation becomes likely to occur.
The dermis is positioned under the stratum basale of the epidermis, and is a layer with a thickness of 2 mm consisting of protein fibers that are constituted of collagen and elastin and substrates. Collagen plays a role in providing taut skin and supporting elastin. Elastin provides elasticity to the skin even though by about 2%. Substrates filling the space between protein fibers are extracellular matrix components such as proteoglycan and hyaluronic acid. Intercellular lipids retain most of the moisture in the dermis. In the skin with youthfulness maintained, production of extracellular matrix components is promoted, and the elasticity, water retention, and tautness of the skin are kept.
Internal factors such as aging, disease, stress, and hormone abnormalities and external factors such as dryness and ultraviolet rays decrease moisture and extracellular matrix (e.g., collagen, hyaluronic acid) from dermal tissues and decrease the functions of the epidermis and dermis of the skin. Decreased skin functions appear as skin wrinkles, decreased elasticity (slackness), or rough skin.
To solve problems regarding beauty including skin aging and rough skin, functional foods for ingestion of various active ingredients and cosmetic compositions have been developed and marketed.
JP 2008-99562 A (Skin-Beautifying Oral Composition) discloses a skin-beautifying oral composition containing royal jelly, isoflavone, and collagen. This disclosure provides food having skin-beautifying effect of improving softness of the skin and fixing up skin texture.
JP 2000-93121 A (Beauty Food) discloses beauty food containing at least one of kuma bamboo grass (Sasa albo-marginata), five-leaf ginseng (Gynostemma pentaphyllum), Siberian ginseng (Acanthopanax senticosus Harms), betaine, and soyasaponin, and a collagen peptide. This disclosure provides a beauty food enabling the restoration of skin tension and gloss and the maintenance of a moist and young bare skin.
JP 11-35445 A (Cosmetic Composition) discloses a cosmetic composition obtained by combining soyasaponin with at least one of hyaluronic acid, salt of hyaluronic acid, collagen, and elastin. This disclosure can enhance skin brightness and maintain healthy and beautiful skin by the synergy of the effect of preventing skin aging by inhibiting the production of subcutaneous lipid peroxide by the combination of soyasaponin with the moisture retention effect by the combination of at least one of hyaluronic acid, salt of hyaluronic acid, collagen, and elastin.
JP 10-29911 A (Cosmetic Material Composition) discloses a cosmetic material composition containing an extract of soybean and adzuki bean powder as essential active ingredients. This disclosure describes a reduction of the abnormality of the skin with increased lipid peroxide in the body, i.e., somber and stained skins, rough skin, and the like.
JP 2004-250372 A (Skin Aging Inhibitor/Improver and/or Rough Skin Inhibitor/Improver Kit) discloses a drink containing an active ingredient effective for beautifying skin and alcohol, and a skin aging inhibitor/improver and/or rough skin inhibitor/improver kit including the drink and a soybean germ extract-containing external preparation. The active ingredient effective for beautifying skin is selected from biological collagen synthesis promoter, abnormal protein remover, female hormone-like agent, and moisturizer.